The sensing efficiency or factor of noble metal nanoparticles is defined as the wavelength shift of the surface plasmon resonance extinction peak position per unit change in the refractive index of the surrounding medium. The sensitivity of different shapes and sizes of gold nanoparticles has been studied by many investigators and found to depend on the plasmon field strength. As a result, the sensitivity factors were found to be larger for hollow nanoparticles than for solid ones of comparable dimensions. This is due to the strong plasmonic fields resulting from the coupling between the external and internal surface plasmon fields in the hollow nanoparticles. In the present paper, the sensitivity factors of a large number of gold nanoframes of different size and wall thickness have been determined by experimental and theoretical computation (using the discrete dipole approximation method). The dependence of the sensitivity factors and the plasmon field strength on the wall thickness and the size of the nanoframes has been determined and is discussed. The sensitivity factors are found to increase linearly with the aspect ratio (wall length/wall thickness) of the nanoframes and are especially sensitive to a decrease in the wall thickness. In comparison with other plasmonic nanoparticles, it is found that nanoframes have sensitivity factors that are 12, 7, and 3 times higher than those of gold nanospheres, gold nanocubes, and gold nanorods, respectively, as well as more than several hundred units higher than those of comparable-size gold nanocages.

Nanocatalysis has attracted great attention in the past two decades, both in heterogeneous solution-phase colloidal reactions [1][2][3] and in heterogeneous supported nanoparticle gas-phase reactions. 3,4 Our group published a Science report early on showing that platinum nanoparticles can be synthesized in a variety of shapes 5 (tetrahedral, cubic, and truncated octahedral). In that report, it was suggested that catalysis could be shape-dependent, and since that time the field has developed extremely rapidly. 6-9 Narayanan and El-Sayed 6 later showed that, indeed, the activation energy of the nanocatalyzed electron-transfer reaction between thiosulfate and hexacyanoferrate III is shape-dependent in colloidal nanocatalysis in which nanocatalysts that have more atoms on edges or corners (i.e., more valency unsaturated atoms) possess more catalytic activity. More recently, shape-dependence was also found to occur in heterogeneous gas-phase nanocatalysis. 7,8 Furthermore, we found that the catalytically active nanoparticles undergo shape changes to the less active nanospheres, causing them to lose some of their activity. [10][11][12] The possible mechanism for shape change and particle growth was recently discussed where the high-resolution TEM images were reported. 13 Previously, 14 tetrahedral platinum nanoparticles have been prepared by the hydrogen reduction technique, and different sizes were synthesized by changing the ratio of the preprepared platinum seed concentration to the Pt +2 concentration. Very recently in a Science report, 3 tetrahexahedral platinum nanocrystals with high index facets, i.e., rich in active valency unsaturated atoms, have been synthesized electrochemically. The authors showed that they are also catalytically very active. These particles were prepared and studied on the surface of an electrode and were not transferable to colloidal solutions.In the present work, tetrahedral platinum nanocrystals (TPNs) were used as seeds for the preparation of a new shape of a colloidal platinum nanostar, without the need for organic solvents, templates, ion replacements, or substrates. The TPNs used were themselves synthesized in large yields by a simple new technique. The lowand high-resolution TEM of the nanostar particles are determined, and the number of arms on each nanostar is found to vary from particle to particle, ranging from a few to over 30. More interesting is that even the largest nanostars are found to form single crystals. This strongly suggests a mechanism of formation involving seeded growth rather than an aggregation or an assembly process of the seed particles. The catalytic activity for these multiarmed nanostar platinum single crystals is examined for the reaction between hexacyanoferrate III (HCFIII) and thiosulfate and found to be higher than the platinum TPN, which are known to be the most active shape of this metal in colloidal solutions.To prepare the tetrahedral platinum nanocrystals (TPNs), 0.0667 g of PVP (MW 360 000) was dissolved in 33 mL of deionized water, 4 drops of...

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There are two main classes of metallic nanoparticles: solid and hollow. Each type can be synthesized in different shapes and structures. Practical use of these nanoparticles depends on the properties they acquire on the nanoscale. Plasmonic nanoparticles of silver and gold are the most studied, with applications in the fields of sensing, medicine, photonics, and catalysis. In this Account, we review our group's work to understand the catalytic properties of metallic nanoparticles of different shapes. Our group was the first to synthesize colloidal metallic nanoparticles of different shapes and compare their catalytic activity in solution. We found that the most active among these were metallic nanoparticles having sharp edges, sharp corners, or rough surfaces. Thus, tetrahedral platinum nanoparticles are more active than spheres. We proposed this happens because sharper, rougher particles have more valency-unsatisfied surface atoms (i.e., atoms that do not have the complete number of bonds that they can chemically accommodate) to act as active sites than smoother nanoparticles. We have not yet resolved whether these catalytically active atoms act as catalytic centers on the surface of the nanoparticle (i.e., heterogeneous catalysis) or are dissolved by the solvent and perform the catalysis in solution (i.e., homogenous catalysis). The answer is probably that it depends on the system studied. In the past few years, the galvanic replacement technique has allowed synthesis of hollow metallic nanoparticles, often called nanocages, including some with nested shells. Nanocage catalysts show strong catalytic activity. We describe several catalytic experiments that suggest the reactions occurred within the cage of the hollow nanocatalysts: (1) We synthesized two types of hollow nanocages with double shells, one with platinum around palladium and the other with palladium around platinum, and two single-shelled nanocages, one made of pure platinum and the other made of pure palladium. The kinetic parameters of each double-shelled catalyst were comparable to those of the single-shelled nanocage of the same metal as the inside shell, which suggests the reactions are taking place inside the cavity. (2) In the second set of experiments, we used double-shelled, hollow nanoparticles with a plasmonic outer gold surface and a non-plasmonic inner catalytic layer of platinum as catalysts. As the reaction proceeded and the dielectric function of the interior gold cavity changed, the plasmonic band of the interior gold shell shifted. This strongly suggested that the reaction had taken place in the nanocage. (3) Finally, we placed a catalyst on the inside walls of hollow nanocages and monitored the corresponding reaction over time. The reaction rate depended on the size and number of holes in the walls of the nanoparticles, strongly suggesting the confinement effect of a nanoreactor.

When the size of a material is reduced to the nanoscale, at or below the characteristic length scale that determines their properties, the material acquires completely new properties. On this length, its properties become sensitive to further changes in size, shape or whether they are hollow or solid. In this perspective article, we first discuss the different experimental techniques used in the synthesis, assembly and handling of colloidal solid or hollow nanoparticles with single and double shells. This is then followed by comparing the experimental and theoretical (DDA and FDTD) results for solid and hollow plasmonic nanoparticles as sensors using two different methods. The first method compares the plasmonic enhancement of the radiative properties of molecules or materials (e.g. in surface enhanced Raman scattering, SERS). The second one is based on the amount of the plasmon peak wavelength shift of the nanoparticle in media with different dielectric functions. In the last section of the perspective, we present a summary of the difference between the solid and hollow nanoparticles in nanocatalysis.We present the results of a number of experiments showing that the superior catalytic properties of hollow nanoparticles are due to catalysis occurring within the cavity of the hollow nanoparticles. Finally, using a femtosecond optical technique, we show that adding a second shell of a stiff metal (like Pt or Pd) to the plasmonic hollow nanoparticles increases their mechanical stability.